The first continental crust on Earth formed more than 3 billion years ago. Likely the first fragments formed by partial melting and re-crystallization of the primordial stagnant crust (formed more than 4.4 billion years ago and covering the entire planet) on a much younger and hotter Earth. Over time those microcontinents were pushed together by convection currents in Earth’s mantle, forming the first supercontinents. Earth’s crust now consisted of a mosaic of tectonic plates, some including continental crust and some plates still retaining the “more primordial” oceanic crust. But geologists are still debating if this growth happened (geologically speaking) very rapidly in just a few million years and ended suddenly, or if it was a slow process needing a billion of years and slowly phasing out .
Now a study published by researchers from Pennsylvania State University contradicts existing theories that suggest the rapid formation of tectonic plates earlier in Earth’s history.
“The dominating theory points to an inflection point some 3 billion years ago, implying we had a stagnant lid planet with no tectonic activity before a sudden shift to tectonic plates,” so lead author Jesse Reimink, assistant professor of geosciences, summarizing the study’s results. “We’ve shown that’s not the case.”
To chart the formation of the Earth’s crust—or the crustal growth curve—the researchers compiled the chemical composition and ages for more than 600,000 samples of the oldest known rocks on Earth. Previous research used single minerals to calculate the crustal growth rate, but according to the authors this method is prone to errors as mineral grains can recrystallize at a later time, adding “phantom time” to the supposed age of crystallization.
Even the rock record is not perfect, but the team used a trick.
“We calculated how much reworking has happened by looking at the composition of igneous rocks in a new way,” Reimink explains.
To do that, the researchers developed a unique method for determining how igneous rocks were reworked and reformed over time and experimentally demonstrating how the same rock could change in different ways over time.
They used these calculations to calibrate the reworking documented in the rock records. Then, they calculated Earth’s crustal growth curve using the new understanding of how the rocks were reformed. They compared the newly calculated curve to the rate of growth gleaned from mineral records by other experts.
The crustal growth curve based on the rock fits a slow formation of continents better than a fast formation as suggested by the minerals alone.
Reimink cautioned that the research improves on what researchers understand, but it’s not the be-all and the end-all for crustal growth research. There are simply too few data points covering the first billion years of Earth’s history. Modern plate tectonics is constantly recycling Earth’s crust, likely fueling the planets bio- and geodivesity, but at the same time destroying the rock record. Rocks older than 3 billion years are very rare and rocks older than 4 billion years are known only from a few remote localities.
The study “A whole-lithosphere view of continental growth” was published in the journal of Geochemical Perspective Letters (2023). Additional material provided by Pennsylvania State University.